1. Technical Field of the Invention
The present invention relates generally to a starter solenoid switch for supplying electric power from a battery to a starter motor in two stages. More particularly, the invention relates to an improved structure of a set of contacts of the starter solenoid switch, which makes and breaks an electrical connection between the battery and the starter motor.
2. Description of the Related Art
In recent years, the number of automobiles, which are equipped with an idle stop system, has increased to meet a demand of improving fuel economy and reducing exhaust gases.
An idle stop system is generally employed to automatically stop the engine when the automobile is stopped for, by way of example, waiting for a traffic light to change or traffic congestion. Therefore, the starter of an automobile with an idle stop system is more frequently operated in comparison with that of an automobile without an idle stop system.
When the starter of an automobile equipped with an idle stop system is designed to crank the engine by bringing a pinion gear thereof into mesh with a ring gear of the engine, the pinion gear and the ring gear may be easily worn down due to the frequent operation of the starter. As a result, the pinion gear of the starter cannot be smoothly brought in mesh with the ring gear of the engine, thus resulting in a difficulty in cranking the engine.
To overcome such a difficulty, Japanese Unexamined Patent Publication No. 2003-293913, an English equivalent of which is U.S. Pat. No. 6,822,544 B2, discloses a magnet switch for a starter.
According to the disclosure, the magnet switch for the starter includes a main contact portion and an auxiliary contact portion, both of which have a fixed and a movable contact and are disposed between a battery and a starter motor in parallel with each other. The main and auxiliary contact portions are so configured that the auxiliary contact portion is closed prior to the main contact portion, thereby supplying electric current from the battery to the starter motor in two stages. Moreover, the fixed contact of the auxiliary contact portion is made of a carbon material that has a high electric resistivity, so that only limited electric current is supplied to the starter motor when only the auxiliary contact portion is closed in the first stage. As a result, in the first stage, the starter motor rotates slowly while a pinion gear of the starter is brought into mesh with a ring gear of the engine, thereby suppressing wear of both the pinion gear and the ring gear. After the pinion gear is completely brought into mesh with the ring gear, the main contact portion is then closed to rotate the starter motor at full speed, thereby reliably cranking the engine.
In such a magnet switch, when assuming that the battery has a nominal voltage of 12 V, the fixed contact of the auxiliary contact portion, which is made of a carbon material, is required to have a resistance in a range of 20 to 100 mΩ. Further, in order to be capable of being arranged inside the magnet switch, the fixed contact of the auxiliary contact portion is required to have a cross sectional area perpendicular to the conducting path of electric current in a range of 50 to 100 mm2 and a length along the conducting path of electric current in a range of 5 to 10 mm.
To meet such requirements, it is necessary for the carbon material of the fixed contact of the auxiliary contact portion to have an electric resistivity in a range of 1,000 to 15,000 μΩcm, and accordingly to contain as more carbon as possible (e.g., 100 wt %).
Further, the fixed contact of the auxiliary contacting portion has two opposite ends disposed along the conducting path of electric current. One end is electrically and mechanically connected, by press fitting, to a metal holder that is connected to the battery; the other end is formed as a contact surface on which the movable contact of the auxiliary contact portion strikes the fixed contact.
However, since carbon has a high electric resistivity, when electric current passes through the auxiliary contact portion, heat may be considerably generated by the fixed contact of the auxiliary contact portion that is made of the carbon material, thereby causing thermal expansion and shrinkage of the metal holder that is connected with the fixed contact. As a result, cracks may be generated in the contacting area between the metal holder and the fixed contact of the auxiliary contact portion due to the difference in coefficient of thermal expansion therebetween, thereby causing a failure of the electrical connection between the battery and the starter motor.
As an alternative, one may consider to embed a copper twisted wire that is connected to the battery in the fixed contact of the auxiliary contact portion, as in the case of making electrical connection for a motor brush. However, the difference in coefficient of thermal expansion between the fixed contact of the auxiliary contact portion that is made of the carbon material and the copper twisted wire would still cause cracks in the contacting area therebetween, thereby causing a failure of the electrical connection.
On the other hand, when the auxiliary contact portion is closed, the movable contact makes contact with the fixed contact at a plurality of contact points on the contact surface. As a result, a high level of heat may be generated at those contact points due to the high electric resistively of the fixed contact made of the carbon material, thereby causing carbon particles to be separated from the contact surface at those points.